Polyolefin compositions with improved properties

ABSTRACT

The invention relates to polyolefin compositions with good impact strength and transparency comprising  
     a) 85-98 wt % based on the total weight of the composition of a heterophasic propylene/α-olefin copolymer comprising a polymer or copolymer of propylene and an α-olefin with 0-15 mol % of the α-olefin as matrix polymer and a propylene/α-olefin rubber copolymer comprising 20-80 mol % of the α-olefin and  
     b) 15-2 wt % based o n the total weight of the composition of an ethylene homopolymer or an ethylene/α-olefin copolymer, the α-olefin in the ethylene copolymer having 4-10 carbon atoms, the ethylene homo-or copolymer having a density lower than 0.925 g/m 3 .  
     processes for producing these compositions and their use. The compositions are particularly useful for food packaging, especially for deep freezing applications.

[0001] The invention relates to polyolefin compositions with improvedimpact strength and improved optical properties. More particularly theinvention relates to polyolefin compositions, especially polypropylenecompositions with improved impact strength at temperatures below roomtemperature, especially at temperatures below 0° C., having improvedoptical properties, especially higher transparency and lower haze.

[0002] The compositions of the inventions are particularly useful forpackaging applications, especially for food packaging for deep freezingapplication.

BACKGROUND OF THE INVENTION

[0003] Polypropylene homopolymers are widely used in packagingapplications. Polypropylene homopolymers show balanced properties andare cheap, easily decomposable polymers, but they show poor transparencyand low impact strength.

[0004] It has been suggested to improve the poor optical properties andthe impact strength of polypropylene homopolymers by adding nucleatingagents and/or clarifiers. Sutiable nucleating agents are for exampletalcum, disorbitol, organic phosphates and the like.

[0005] Nevertheless the desired properties of propylene homopolymerscannot be improved to the extent desired, namely to show high impactstrength at low temperatures.

[0006] Compositions with improved impact strength and improved opticalproperties are for example propylene/α-olefin random copolymers, whereinthe α-olefin is ethylene or an α-olefin with 4-10 carbon atoms. Althoughtransparency is higher and impact strength is improved considerably atambient temperature, impact strength at lower temperature, i.e. 0° C.and below is still not satisfying. Propylene/α-olefin random copolymershave a ductile to brittle transition temperature between −5° C. and 5°C. depending on the monomer content and therefore impact strength atlower temperatures is still very poor.

[0007] To improve the impact strength at lower temperatures heterophasicpolymer compositions have been suggested. Heterophasic polymers arepolymers having a matrix phase (phase 1) and a second phase. The matrixphase is usually a polypropylene homopolymer or polypropylene/α-olefincopolymer phase and the second phase is usually a propylene/α-olefinrubber polymer.

[0008] The polymers of the second phase have low glass transitiontemperatures, usually below −30° C. Therefore impact strength of suchheterophasic systems is rather satisfying, even for deep freezingapplications. A major drawback is the poor transparency due mostly dueto big rubber particles in the heterophasic system.

[0009] Several attempts have been made to improve the transparency ofheterophasic polymer systems.

[0010] EP-B 0 373 660 discloses heterophasic polymer systems withimproved impact strength and improved transparency an elastomericpropylene/ethylene as matrix phase and a phase 2 beingpropylene/α-olefin rubber, wherein the intrinsic viscosity ratio betweenphase 1 and phase 2 has to be a defined value, namely 0.2. Therefore theheterophasic system of the disclosure of EP B 0 373 660 is restricted tovery special combinations of phase 1 and 2 polymers.

[0011] EP-A 0 814 127 discloses embrittlement resistant polyolefincompositions and flexible articles thereof based on propylenehomopolymers or propylene/α-olefin copolymers and elastomeric copolymersof ethylene with propylene or butene-1. The elastomeric copoylmeresenhance the flexibility without affecting the clarity of the polymercomposition, which is not at least satisfying for packagingapplications, although these compositions are claimed to be used for theproduction of medical articles and food packaging material.

[0012] EP B 0 593 221 discloses polypropylene resin compositionscomprising polypropylenes of defined MFR and ethylene/α-olefincopolymers with good balance between impact strength and rigidity andhigh moldability useful for automotive parts. These compositions do nothave good optical properties, especially low transparency and high haze.

OBJECT OF THE INVENTION

[0013] It is an object of the invention to provide polypropylenecompositions for packaging applications with improved impact strength atambient and low temperatures having improved optical properties,especially good transparency and low haze.

[0014] A further object of the invention is a process for producingpolypropylene compositions for packaging applications with improvedimpact strength at low temperatures and good optical properties.

[0015] It is still a further object of the invention to provide articlesmade of these polypropylene compositions useful for packagingapplications, especially for food packaging for deep freezingapplications.

BRIEF DESCRIPTION OF THE DRAWING

[0016] The object of the invention has been solved by providingheterophasic polymer systems which are modified by a modifier whichshows improved impact strength as well as improved optical properties.

[0017] The invention therefore relates to polyolefin compositions withgood impact strength and transparency comprising

[0018] a) 85-98 wt % based on the total weight of the composition of aheterophasic propylene/α-olefin copolymer comprising a polymer orcopolymer of propylene and an α-olefin with 0-15 mol % of the α-olefinas matrix polymer and a propylene/α-olefin rubber copolymer comprising20-80 mol % of the α-olefin

[0019] b) 15-2 wt % based on the total weight of the composition of anethylene homopolymer or an ethylene/α-olefin copolymer, the α-olefin inthe ethylene copolymer having 4-10 carbon atoms, the ethylene homo-orcopolymer having a density lower than 0.925 g/m³.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Component a) is preferably a heterophasic propylene/α-olefincopolymer comprising a polymer or a copolymer of propylene with 0-15 mol% of an α-olefin as matrix polymer (phase 1 polymer) and apropylene/α-olefin rubber copolymer comprising 20-80 mol % of theα-olefin.

[0021] Preferably the molecular weight of the propylene/α-olefin rubberis equal or lower than the molecular weight of the propylene homopolymerresp. the polypropylene/α-olefin copolymer.

[0022] The heterophasic polymer of component a) may be produced bymultistage process polymerisation of polypropylene or polypropylene andα-olefin such as bulk polymerisation, gas phase polymerisation, slurrypolymerisation, solution polymerisation or combinations thereof usingconventional catalysts. Those processes are well known to one skilled inthe art.

[0023] A preferred process is a combination of a bulk slurry loopreactor(s) and gas phase reactor(s). The matrix polymer can be madeeither in loop reactors or in a combination of loop and gas phasereactor.

[0024] The polymer produced in this way is transferred into anotherreactor and the propylene/α-olefin rubber is polymerised. Preferablythis polymerisation step is done in a gas phase polymerisation.

[0025] A suitable catalyst for the polymerisation of the heterophasiccopolymer is any stereospecific catalyst for propylene polymerisationwhich is capable of polymerising and copolymerising propylene andcomonomers at a temperature of 40 to 110° C. and at a pressure form 10to 100 bar. Ziegler Natta catalysts as well as metallocene catalysts aresuitable catalysts.

[0026] One skilled in the art is aware of the various possibilities toproduce such heterophasic systems and will simply find out a suitableprocedure to produce suitable heterophasic polymer systems which areused in the present invention.

[0027] The matrix polymers of the heterophasic polymers of component a)may have a ratio of matrix polymer (polymer of phase 1) to polymer ofphase 2 of about 97:3 to 80:20, preferably 95:5 to 90:10.

[0028] The heterophasic polymer shows a melt flow rate of about 0.1 to200 g/10 min, preferably 0.2 to 50 g/10 min, more preferable 0.3 to 20g/10 min according to ISO 1133. The ethylene content of the matrixpolymer may be up to 7 mol % and the overall ethylene content of theheterophasic polymer of component a) may be up to 30 mol %, preferablyup to 15 mol %.

[0029] The polymer of component a) preferably has a flexural modulus ofhigher than 500 MPa (DIN 53457).

[0030] Component b) is an ethylene homopolymer or an ethylene/α-olefincopolymer the α-olefin having 4-10 carbon atoms.

[0031] For example component b) may be a low density ethylenehomopolymer or a copolymer of ethylene with for example vinylacetate orbutylacrylate or the like. The amount of component b) is 2-15 wt % basedon the total weight of the composition, preferably 3-10 wt %, mostpreferably 4-7 wt %.

[0032] Preferably the density of the low density ethylene homo- orcopolymer (component b)) is lower than 0.925 g/cm³ according to ISO1183. More preferably the density of the low density ethylene homo- orcopolymer is lower than 0.920 g/cm³. Most preferably the density is0.890-0.920 g/cm³.

[0033] Preferably the ethylene-homo- or copolymer (component b)) shouldhave a substantially higher melt flow rate (MFR) (temperature: 190° C.;load: 2.16 kg) than the heterophasic polymer of component a) undermelting and blending conditions. Preferably the melt flow rate ofcomponent b) is at least two times, more preferably at least five timeshigher than the melt flow rate of component a). MFR's of component b)are determined at a temperature of 190° C. and a load of 2.16 kg. MFR'sof component a) are determined at a temperature of 230° C. and a load of2.16 kg.

[0034] Those ethylene homo- or copolymers may be produced in a highpressure tubular process or in a high pressure autoclave process.

[0035] Alternatively, a linear low density type ethylene copolymer maybe produced in a low pressure process typically with a Ziegler typecatalyst. Further the polymers of component b) may be produced using ametallocene catalyst. The polymers of component b) produced by usingmetallocene catalysts are typically elastic copolymers of ethylene andbutene or- octane having a density of preferably 0.890-0.915 g/cm³.

[0036] The compositions of the present invention may further compriseconventional additives, such as antioxidants, stabilisers, acidscavengers, clarifying agents, coloring agents, anti-UV agents,nucleating agents, antistatic agents, slip/mould release agents,fillers, like nanofillers, etc. Typically these additives may be presentat less than 2 wt % each, more preferably less than 0.5 wt % relative tothe total weight of the composition.

[0037] Examples of such conventional additives include Irganox 1010 andIrgafos 168 (commercially available stabilizers form Ciba SpecialityChemicals), calcium stearate and synthetic hydrotalcite (e.g. DHT-4Afrom Kyowa Chemical Industry) and1,2:3,4-di(ethylbenzylidene)sorbitol—EBDS (e.g. NC-4 from Mitsui Toatsuand 1,3:2,4 bis(3,4-dimethylbenzylidene)sorbitol—DMBDS (e.g. Millad 3988from Milliken Chemicals).

[0038] The polypropylene composition of the present invention ispreferably clarified. Clarified polypropylenes ca be produced forexample by adding clarifying or nucleating agents, for examples sorbitolderivatives such as EDBS, MDBS (1,3:2,7-di(methylbenzylidene)sorbitoland DMDBS, phosphate salts, such as for example sodium2,2′-methylenebis(4,6-di-tertiarbutylphenyl)phosphate,polyvinylcylohexane etc. Typically such clarifying or nucleating agentsmay result in haze levels after injection moulding of lower than 60%,preferably lower than 40% in 2 mm injection moulding sheets.

[0039] The polyolefin compositions of the present invention are producedby mixing the heterophasic polymer of component a) with component b)optionally adding conventional additives and/or stabilisers and/orfillers.

[0040] Preferably, mixing is done by melt blending in an extruder oranother melt blending unit, preferably in a twin screw extruder usuallyfollowed by pelletisation. The components may be melt blended as such,or melt blending can be done in the presence of a peroxide component forincreasing the melt flow rate of the composition. Then the peroxide,preferably an organic peroxide, which is suitable for degradation ofpolypropylene (visbreaking) is fed to the blending unit together withcomponents a) and b) or the peroxide may be fed separately into the meltblending unit.

[0041] The compositions of the present invention show improved impactstrength, especially at temperatures below 0° C. and improved opticalproperties, especially high transparency and low haze.

[0042] The polypropylene composition of this invention can be furtherconverted to an end product by using normal conversion techniques, suchas injection moulding, compression moulding, blow moulding (extrusion orinjection stretch blow moulding), extrusion (film, sheet, pipe, tuber,profile extrusion), film blowing, thermoforming and the like. Preferablyend products are packaging containers made by injection moulding, blowmoulding or thermoforming, or packaging films made by film extrusion.

[0043] The products are particularly suitable for food packagingapplications, especially for deep freezing applications.

[0044] It is a still further object of the invention to provide newarticles with good impact strength and good optical properties,especially good transparency an low haze.

[0045] This object is achieved by using a polyolefin compositioncomprising

[0046] a) 85-98 wt % based on the total weight of the composition of aheterophasic propylene/α-olefin copolymer comprising a polymer orcopolymer of propylene and an α-olefin with 0-15 mol % of the α-olefinas matrix polymer and a propylene/α-olefin rubber copolymer comprising20-80 mol % of the α-olefin and

[0047] b) 15-2 wt % based on the total weight of the composition of anethylene homopolymer or an ethylene/α-olefin copolymer, the α-olefin inthe ethylene copolymer having 4-10 carbon atoms, the ethylene homo-orcopolymer having a density lower than 0.925 g/m³,

[0048] for the production of articles with good impact strength goodtransparency.

EXAMPLES Heterophasic Polymers

[0049] The heterophasic polymers were produced in a two step process. Inthe first stage a random propylene/ethylene copolymer was polymerised(polymerisation in liquid propylene) and in the second stage apropylene/ethylene rubber was polymerised. The ratio between thepropylene/ethylene random copolymer and the propylene/ethylene rubberwas 92:8.

[0050] For visbreaking from the basic melt flow rate to a melt flow rateof about 10-15 g/10 min Di-tertiary-butylperoxide was used.

[0051] The compositions and the additives were compounded in a twinscrew extruder at a temperature of 250° C. The strands were quenched incold water and pelletized.

[0052] The characteristics of the heterophasic polymers are given intable 1. TABLE 1 MFR2 XCS C2 MFR2 XCS C2 i..V. matrix matrix matrixtotal total total XCS C3/XCS polymer [g/10′] [wt %] [mol %] [g/10′] [wt%] [mol %] [ml/g] [wt %] polymer 1 1.09 7.2 5.6 1.22 12.2 8.5 1.38 66.0polymer 2 1.13 6.7 6.3 1.27 12.0 9.8 1.65 65.0 polymer 3 1.13 6.7 6.31.19 12.1 9.2 1.59 66.0 polymer 4 1.05 6.6 6.0 1.12 13.3 8.4 1.70 68.5

[0053] The polymers were mixed with conventional additives (0.05%Irgafos 168, 0.05% Irganox 1010, 0.1 Ca-stearate, 0.06% Glycerolmonostearate, 0.18% Millad 3988) in an intensive mixer (Henschel mixer)for 20 seconds.

Examples 1-4

[0054] Polymers 1-4 of table 1 (component a) )were mixed with 5% Exact2M048, a commercial metallocene catalyst based polyethylene grade ofMFR(90° C., 2.16 kg) 10 g/10 min, density 902 g/cm³; DexPlastomers(component b)) was added.

[0055] For visbreaking from the basic melt flow rate to a melt flow rateof about 10-15 g/10 min Di-tertiary-butylperoxide was used.

[0056] The compositions and the additives were compounded in a twinscrew extruder at a temperature of 250° C. The strands were quenched incold water and pelletized.

[0057] Injection molded test bars were produced and the notched impactstrength according to ISO 179 was measured.

[0058] For determining optical properties, especially transparency andhaze injection molded plaques (60*60*2 mm) were produced. The opticalproperties were measured according to ASTM D-1003-92.

Comparative Examples 1-4

[0059] Heterophasic polymers of table 1 were used as comparativecompounds.

[0060] Impact strength and optical properties were determined asdescribed.

[0061] The results are given in table 2. TABLE 2 NIS/+23° C. IS/−20° C.179 179 MFR 1eA/+23° C. 1eU/−20° C. haze example polymer modifier[g/10′] [kJ/m²] [kJ/m²] [%] example 1 polymer 1 5% Exact 2M048 12.7 11.677.5 28.0 example 2 polymer 2 5% Exact 2M048 12.4 13.2 131.7 39.4example 3 polymer 3 5% Exact 2M048 12.5 11.8 125.9 38.5 example 4polymer 4 5% Exact 2M048 12.0 11.1 130.3 43.5 comp. Example 1 polymer 113.8 9.4 62.9 36.0 comp. Example 2 polymer 2 13.6 10.5 88.9 54.0 comp.Example 3 polymer 3 13.4 9.3 75.8 53.6 comp. Example 4 polymer 4 13.08.8 70.4 60.6

Examples 5-6

[0062] Selected heterophasic polymers of table 1 (component a ) weremixed with 5% Borealis CA9150 (commercial available LDPE grade ofMFR(190° C., 2.16 kg) 15 g/10 min, density 915 g/cm³, (component b)).

[0063] Comparative example 5 and 6 were prepared without addingcomponent b).

[0064] The compositions were treated as described above, impact strengthand optical properties were determined as described above.

[0065] The results are given in table 3. TABLE 3 NIS/+23° C. IS/−20° C.179 179 MFR 1eA/+23° C. 1eU/−20° C. Haze example polymer modifier[g/10′] [kJ/m²] [kJ/m²] [%] example 5 polymer 2 5% Borealis CA 13.5 10.5108.3 40.8 9150 example 6 polymer 4 5% Borealis CA 12.5 10.0 115.8 44.29150

1) Polyolefin compositions with good impact strength and transparencycomprising a) 85-98 wt % based on the total weight of the composition ofa heterophasic propylene/α-olefin copolymer comprising a polymer orcopolymer of propylene and an α-olefin with 0-15 mol % of the α-olefinas matrix polymer and a propylene/α-olefin rubber copolymer comprising20-80 mol % of the α-olefin and b) 15-2 wt % based on the total weightof the composition of an ethylene homopolymer or an ethylene/α-olefincopolymer, the α-olefin in the ethylene copolymer having 4-10 carbonatoms, the ethylene homo-or copolymer having a density lower than 0.925g/m³. 2) Polyolefin compositions according to claim 1, comprising a)90-97 wt % of a heterophasic propylene/α-olefin copolymer and b) 10-3 wt% of an ethylene homopolymer or an ethylene/α-olefin copolymer. 3)Polyolefin compositions according to one of claims 1 to 2, wherein thematrix polymer of the heterophasic polypropylene/α-olefin copolymer is apropylene/ethylene copolymer and the phase 2 polymer is apropylene/ethylene rubber. 4) Polyolefin compositions according to oneof the claims 1 to 3 wherein the ratio of the matrix polymer to phase 2polymer is about 97:3 to 80:20. 5) Polyolefin compositions according toone of the claims 1 to 4 wherein component b) is selected from the groupof low density ethylene homopolymers and metallocene basedethylene/α-olefin copolymers. 6) Polyolefin compositions according toone of the claims 1 to 5, wherein the ethylene content of the polyolefincomposition is up to 15 mol %. 7) Polyolefin compositions according toone of the claims 1 to 6, wherein the melt flow rate of the polymercomposition is 0.1-200 g/min, preferably 0.2-50 g/10 min, mostpreferably 0.5-20 g/min. 8) Polyolefin compositions according to one ofthe claims 1 to 7, wherein the melt flow rate (190° C., 2.16 kg) ofcomponent b) is higher, preferably at least two times and morepreferably at least 5 times higher than the melt flow rate (230° C.,2.16 kg) of component a) under melting and blending conditions. 9)Polyolefin compositions according to one of the claims 1 to 8 furthercomprising conventional additives and/or stabiliser and/or fillers. 10)Process for producing polyolefin compositions according to one of theclaims 1 to 9, wherein the heterophasic polymer of component a) isproduced in a multi stage process and mixed with the ethylenehomopolymer or the ethylene/α-olefin copolymer. 11) Process according toclaim 10, wherein components a) and b) are mixed by melt blending in thepresence or in the absence of peroxides. 12) Use of the polymercompositions of one of the claims 1 to 9 for producing products forpackaging applications by injection moulding, compression moulding, blowmoulding, extrusion (film-, sheet-, pipe-, tube-, profile extrusion),film blowing, and thermoforming. 13) Use of a polyolefin compositioncomprising a) 85-98 wt % based on the total weight of the composition ofa heterophasic propylene/α-olefin copolymer comprising a polymer orcopolymer of propylene and an α-olefin with 0-15 mol % of the α-olefinas matrix polymer and a propylene/α-olefin rubber copolymer comprising20-80 mol % of the α-olefin and b) 15-2 wt % based on the total weightof the composition of an ethylene homopolymer or an ethylene/α-olefincopolymer, the α-olefin in the ethylene copolymer having 4-10 carbonatoms, the ethylene homo-or copolymer having a density lower than 0.925g/m³, for the production of articles with good impact strength goodtransparency.